I. Field of the Invention
The present application is related to copending U.S. patent application Ser. No. 07/433,030, filed Nov. 7, 1989, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM" (now U.S. Pat. No. 5,101,501) and as such also relates cellular communications systems. More specifically, the present invention relates to a novel and improved system for controlling the handoff in communications of a mobile station between cell base stations or sectors thereof in a code division multiple access (CDMA) cellular communications system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, the disclosure thereof incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations, or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In the conventional cellular telephone systems the available frequency band is divided into channels typically 30 KHz in bandwidth while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors of that cell.
In the conventional cellular telephone system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between two cells. The handoff from one cell to another is initiated when the reciever in the cell base station handling the call notices that the received signal strength from the mobile telephone falls below a predetermined threshold value. A low signal strength indication implies that the mobile telephone must be near the cell border. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station receives the mobile telephone signal with better signal strength than the current base station.
The system controller in response to the current base station inquiry sends messages to the neighboring base stations with a handoff request. The base stations neighboring the current base station employ special scanning receivers which look for the signal from the mobile telephone on the specified channel. Should one of the neighboring base stations report an adequate signal level to the system controller, then a handoff will be attempted.
Handoff is then initiated when an idle channel from the channel set used in the new base station is selected. A control message is sent to the mobile telephone commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the second base station.
In the conventional system a call will be discontinued if the handoff to the new base station is unsuccessful. There are many reasons that a failure in handoff may occur. Handoff can fail if there is no idle channel available in the neighboring cell for communicating the call. Handoff can also fail if another base station reports hearing the mobile telephone in question, when in fact this base station actually hears a different mobile telephone using the same channel in a completely different cell. This reporting error will result in the call being switched to a wrong cell, typically one in which signal strength is insufficient to maintain communications. Furthermore should the mobile telephone fail to hear the command to switch channels, the handoff will fail. Actual operating experience indicates that handoff failures occur frequently which questions the reliability of the system.
Another common problem in the conventional telephone system occurs when the mobile telephone is near the border between two cells. In this situation the signal level tends to fluctuate at both base stations. This signal level fluctuation results in a ping-ponging situation in which repeated requests are made to hand the call back and forth between the two base stations. Such additional unnecessary handoff requests increase the possibility of the mobile station incorrectly hearing the channel switch command or failing to hear the command at all. Furthermore, the ping-ponging situation raises the possibility that the call will be discontinued if it is inadvertently transferred to a cell in which all channels are currently in use and thus unavailable for accepting the handoff.
In the co-pending parent application a method and system are disclosed for providing a communication with the mobile station through more than one cell base stations during the handoff. In this environment communication between the mobile station and the other user is uninterrupted by the eventual handoff from the base station corresponding to cell from which the mobile station is exiting to the base station corresponding to cell from which the mobile station is entering. This type of handoff may be considered as a "soft" handoff in communications between cell base stations with the mobile wherein two or more base stations or sectors of a base station transmit concurrently to the mobile station. Similar are the techniques for a handoff between a sector of one cell and another cell, and a handoff between sectors of a same cell base station for a sectorized cell.
The present invention provides a substantial improvement over current cellular telephone systems with respect to mobile station handoff. The make-before-break handoff mechanism of the present invention is a significant improvement in overall system reliability with lower service disruption. The implementation of a base station diversity mode provides further improvements over conventional cellular telephone systems by providing additional system reliability and quality in communications.
It is therefore an object of the present invention to provide in a cellular communication system an improvement in call handoff in communications of a mobile station between base station so as to provide greater service reliability and quality.